Cartridge based tape and disk data storage devices have been in use in the computer industry for several decades. Primarily, the cartridge based tape storage devices have been used as sequential access devices, whereby new files are added to the tape by appending them to the last file stored on the tape. During that time, a number of tape cartridge styles emerged. One popular cartridge style is based on a design that is disclosed in U.S. Pat. No. 3,692,255, to Von Behren. That design contains two rotatable reels that are fixed within a rectangular housing. A length of tape is wound around the reels along a predetermined tape path, which extends along a front peripheral edge of the cartridge and across a tape access opening. A drive belt extends around drive belt rollers and contacts a portion of the tape on each reel to move the tape back and forth between the reels and across the tape access opening. A drive puck, positioned near the inside front of the cartridge, contacts a drive roller, which provides a mechanism to move the drive belt.
Tape cartridges and tape drives have become an increasingly important feature in computer systems. The popularity of tape cartridges is driven in part by the large storage capacities and low cost of storage they provide. In general, the storage capacity of a particular tape cartridge is dictated by a variety of factors including the length of tape, the width of the tape, the materials used to produce the tape and the recording density of the tape. Even with the cost and capacity advantages offered by tape storage solutions, the tape drive has primarily found use in computer systems as a back-up device, in which duplicates of files that were originally stored to a random access storage device, such as a hard disk drive, are stored for sequential access on a tape cartridge.
The popularity of these tape drive and cartridges have spawned several tape drive and cartridge standards. One popular tape cartridge and tape drive standard is defined in the specification entitled "Serial Recorded Magnetic Tape Minicartridge for Information Interchange," QIC-3020-MC, Revision H, Mar. 20, 1996. This standard defines a variety of important features related to tape cartridges such as tape width, recording format, track format, segment format, and so on.
Typically, tape drives are used in hard disk drive back-up procedures. Files contained on a hard disk drive are stored on tracks on the recording tape in a sequential fashion in accordance with the format of the tape. That is, each new file is appended after the last file stored on the tape. Additionally, files that are accessed infrequently can be stored on tape and permanently removed from the hard drive. As a result, valuable hard disk file space is available for more immediate file needs.
In the event of a loss of files on the hard drive, the files can be retrieved from the tape and restored to the hard disk drive. To perform this task, the tape within the cartridge must be appropriately positioned to retrieve such files. If a user desires to restore a few selected files, it is possible that the tape will be repositioned several times during the restore procedure. Conventional tape positioning techniques include detecting and counting erased gaps between formatted segments (i.e., inter-segment gaps) on the tape. This technique is described in the document entitled, "Command Set Interface Specification for Flexible Disk Controller Based Mini Data Cartridge Tape Drives," QIC-117, Revision J, Aug. 28, 1996. As described in the standard, host software passes arguments to the tape drive indicating the relative number of segments to traverse in either the forward or reverse direction. When the arguments are received by the tape drive, the drive moves the tape at high speed while detecting the inter-segment gaps until the number of segments traversed satisfies the request of the host software. In accordance with the standard, to properly position the tape, the number of segments traversed is one greater than the number of segments requested by the passed arguments.
A disadvantage of this method is that the erased gaps must be accurately detected and counted during the high speed movement of the tape. Often, gaps are not detected because of track-to-track interference, or the failure of the recording head to accurately track the recording tracks on the tape during high speed movement due to, for example, misalignment of the head. Further, the number of segments traversed may be affected by the tape drive motor start and stop times. To illustrate the potential track-to-track interference problem, the QIC-3020-MC standard calls for a quarter-inch tape to have 40 parallel tracks. The track position tolerances are to be within 0.0011 inches (.+-.0.0279 mm) of their define positions, and the nominal track spacing is 0.006 inch (0.1524 mm). Thus, a slight misalignment of the tape within the cartridge or of the head in the tape drive may cause severe degradation of the signal and an increase in the track-to-track interference.
Inaccurate detection of the erased gaps may cause delays or errors in the restoration process, and in the worst case scenario, the total failure of the tape to be properly positioned. Such delays, errors and failures are very often inconvenient and frustrating to users of tape drives who are typically restoring important inaccessible or destroyed data files under stressful situations. Therefore, there is a need for an improved tape positioning system that offers advantages over the prior art. The present invention provides such a tape positioning system.